Overwintering

Overwintering tender perennials

If you grow tender perennials, you need to overwinter them, rather than let them die and waste money. Here’s a run-down of what to do with the most common half-hardy perennials:

I kept my Canna dry in their pots over winter and they have a massive start on last year

Echiums overwintering in the conservatory but this has proved not necessary in the past two years

Cobaea scandens flowering under fleece in the greenhouse in February

CANNAS: Lift after leaves have been frosted. Cut back stems to 5cm from the base. Store only undamaged fleshy rhizomes. Remove loose soil, then store in trays of sand or vermiculite, with the crown of the plant just showing. Keep just moist in a frost-free greenhouse or conservatory. Container-grown plants can be stored in their pots after cutting back.

ECHIUM: E. pininana is a biennial/triennial. After keeping seven inside in winter 2013/14 (and falling victim to aphids) – the three outside in pots emerged best, with fleece bags over them – and the outside ones flowered. The other five spent last winter outside, in gritty soil. They’ll survive odd nights down to -4ºC. Remove all dead/marked leaves and reduce watering. By late April, all were elongating and starting to set flower buds – then they’ll die.

COBAEA SCANDENS (cup-and-saucer vine); Cardinal climber; Spanish Flag: These climbers must have a winter temperature of above 5ºC. Too dry, and they can fall prey to red spider mite or die. Quite tricky to get the balance right – only managed it with a cup-and-saucer vine.

DECIDUOUS AGAPANTHUS, GLOBE ARTICHOKES, LILIES: All benefit from a thick, dry mulch and good drainage when left in the ground – straw, compost, chipped bark or well-rotted manure is suitable.

Cucamelon tubers – watch out for rot

Bananas and Aeoniums on BioGreen heat mat

Emo fig reluctantly breaking dormancy as it wishes it was dead

CUCAMELONS: Cucamelons are perennials and produce radish-like roots. As they slow down in autumn, expose the roots to see if they have produced tubers. Store these in slightly moist compost in a frost-free place. Plant into pots early to mid-April under glass. Another one difficult to get the balance right – most of my tubers rotted. Have backup emergency seeds.

BANANAS: Kept dry over winter, plants can remain dormant and will survive 5ºC, but won’t fruit. Plants are best overwintered at temperatures of about 10ºC in a heated greenhouse. For fruit, a winter minimum of 15ºC is recommended. I use a tomato house with the heated pad from my propagator to keep night temperatures up. They continue to grow but not at their usual phenomenal rate.

FIGS: My ’emo fig’ as it sulks a lot, lives in a cold greenhouse. In autumn, reduce watering and remove any remaining leaves and fruit. It needs to be dark and dry, and just above freezing – an unheated shed or garage is ideal.

The usual advice is to keep it dark. I didn’t bother. Keep the soil almost completely dry – moist soil can rot the roots. It has awakened.

YUCCAS/CORDYLINES: Overwinter more tender species (usually red, purple or variegated forms) in a greenhouse providing minimum temperatures of 5°C. Keep on the dry side.

Potted guide: overwintering

Watch out for pests – you should thoroughly clean out your overwintering spots before moving plants into them, as there are bound to be some pests you miss.

Place yellow sticky fly traps every 1m to catch any you miss and inspect plants at least twice a week.

Bedding plants

DAHLIAS/TUBEROUS BEGONIAS: When the foliage has been blackened by frost, cut off the flowering stems 5cm from the base and trim away thin roots. Use a fork to lift plants, taking care not to damage the tubers.

Put soil-free tubers upside down in a cool place for a few weeks to dry. Bury in trays filled with dry sand, soil or compost, leaving only the flower stalks exposed. Place in a cool but frost-free place.

Dahlia tubers potted up after making it through the winter

How to kill geraniums – keep them in humidity this high in November

GLADIOLI: Wait until leaves have died back, then dig up the corms. Remove any loose dirt. Leave corms on top of the soil for two days, then transfer them to a box and place it in a warm, dry place with good air circulation, for two weeks.

Gladioli form a new corm on top of last year’s old one, so discard the old one and cut the dead foliage off. Place them in single layers in cardboard boxes with newspaper in between and keep in a cool, dry spot at about 5ºC.

PELARGONIUMS (geraniums): If they’re warm enough and in good light, they will continue to grow and flower. Mine generally do until November, then I reduce watering.

Before bringing them under glass, clean off dead leaves or dying flowers and check for hidden pests. Try to keep the atmosphere dry, but not the roots – the plants do not go into dormancy and need some moisture.

Ventilation is important, to prevent against mould and rotting. They only need to be frost-free, although above 5ºC is best – if frost damages the stems, the plant will die.

Archives

Subscribe to Blog via Email

Mandy Watson is a freelance journalist and an incurable plantaholic. MandyCanUDigIt grew from the tiny seed of a Twitter account into the rainforest of information you see now. Gardening columnist (Sunderland Echo, Shields Gazette, Hartlepool Mail), Teesdale Mercury Magazine editor. Attracted by the rebellious, exotic and nerdy. Passionate about northern England and gardens everywhere. Falls over a lot.